1. Field of the Invention
The present invention relates to an electron microscope and method of adjusting it.
2. Description of Related Art
Generally, electrons released from a field-emission electron gun contain a fluctuation portion representing several percent of the total amount, because gas is adsorbed onto the emitter surface and adsorbed gas and ions migrate, varying the work function of the metal surface and because ion bombardment varies the metal surface morphology. Therefore, where a field-emission electron gun is used in a scanning electron microscope, emission noise on the image is removed by placing a detector for noise cancellation in the electron optical column, detecting near electrons forming a probe to thereby form an electron detection signal, and dividing a signal released from the sample by the electron detection signal. Such an electron microscope is disclosed, for example, in JP-A-5-307942.
FIG. 7 shows the configuration of a scanning transmission electron microscope (STEM), 100, having a general noise canceling function. This microscope 100 is configured including an electron beam source 111, a noise canceling aperture 112, a lens 113, a detector 115, a preamplifier 120, a noise canceling circuit 130, and a processing section (CPU) 140. The electron beam released from the electron beam source 111 is partially cut off by the noise canceling aperture 112 and then converged onto a sample 114 by the lens 113. The beam transmitted through the sample 114 is partially detected by the detector 115. The output signal from the detector 115 is amplified by the preamplifier 120 and then the emission noise component of the signal is removed by the noise canceling circuit 130. The processing section 140 converts the analog output signal from the noise canceling circuit 130 into digital form, averages or otherwise processes the output signal, creates image data, and sends the data to a personal computer (PC) 200. The computer 200 receives the image data, creates an image of the sample 114, displays the image on a display unit, and stores or otherwise processes the image. If the current iP (t) hitting the sample 114 and the output iA (t) from the noise canceling aperture 112 are in a proportional relationship, and if the offset component added by the preamplifier 120 is correctly subtracted by the first stage of the noise canceling circuit 130, then contrast produced by the emission noise in an STEM image can be reduced. If the subtraction of the offset component is not correct, then it follows that the difference (constant) divided by IA (t) is added to the detection signal (output signal), thus increasing the noise in the STEM image. The offset component (G×O) set into the noise canceling circuit 130 has been calculated in an analog or digital manner from the offset signal (O) set into the preamplifier 120.
In this way, in the past, the amount of offset set into the noise canceling circuit has been computed based on information about a signal set into a detector. Where an external detector is used, a sufficient amount of information about the offset of the detector may not be obtained. This makes it difficult to perform the noise canceling function effectively. Furthermore, the output signals from the detector and preamplifier are varied due to the effects of temperature variations. This varies the amount of offset to be set into the noise canceling circuit.